The present invention relates to planar optical waveguides and, more particularly, to planar optical waveguides that employ optical layers engineered to minimize or eliminate strain-induced birefringence in such planar optical waveguides.
Presently, optical devices that perform useful and necessary functions in optical communication systems are typically built within a block of transparent material, such as silica, silicon or lithium niobate, among others, using so-called xe2x80x9cplanar optical waveguides.xe2x80x9d Planar optical waveguides guide light along defined paths through the optical device using a region of material of higher refractive index (the core) than the surrounding material (the cladding). Such planar optical waveguides are conventionally formed by depositing a base layer of silicon dioxide (lower cladding layer) on a silicon substrate, followed by the deposition of a doped silica layer to form the waveguide core which confines the light in the same way a fiber does. Using standard lithographic techniques, the doped silica core layer is patterned to form a rectangular cross-sectional core. Following this latter patterning, an additional layer of silica is deposited to act as a top cladding layer.
During fabrication, however, any introduced strain due to the different thermal expansion between silicon and silica induces birefringence in the waveguide core. Unfortunately, this stress-induced birefringence causes the two orthogonal polarization modes of the light in the waveguide core to travel at slightly different propagation speeds, adversely affecting the transmission of the light and the performance of the optical device.
As such, various solutions have been proposed for reducing, or eliminating the stress-induced birefringence in planar optical waveguides. In one approach, deep grooves are judiciously etched adjacent to the waveguide to release the strain. See, for example, xe2x80x9cPolarisation Insensitive Wavelength Multiplexers Using Stress Release Grooves,xe2x80x9d Nadler et al., ECOC""98, pp. 20-24, September 1998, Madrid, Spain. This technique, however, is believed to significantly increase manufacturing costs.
In another approach, doped silica substrates rather than silicon substrates, functioning as the lower cladding layer, are employed to support the doped silica waveguide structures. To reduce or eliminate the strain-induced birefringence, the doped silica substrate is made to have a coefficient of thermal expansion approximating the temperature coefficient of the doped silica substrate. See, for example, U.S. Pat. No. 5,483,613, which is commonly assigned and incorporated herein by reference. As stated therein, the reasoning behind this latter technique is that xe2x80x9cthe doped silica substrate and the waveguide element layer contract the same amount as they cool, resulting in substantial elimination of thermally-induced strain caused by different degrees of contraction.xe2x80x9d Col.3:3-6. In another approach, U.S. Pat. No. 5,930,439 discloses discovering empirically that by making the coefficient of thermal expansion of the upper cladding layer close to that of the substrate, it was possible to achieve low polarization sensitivity in conventional waveguide structures.
Unfortunately, it has not been recognized that the above latter techniques are based on an analysis that does not accurately account for factors affecting the strain in the waveguide core, and hence misleading when used for the purpose of eliminating the strain-induced birefringence discussed herein above.
In accordance with the teachings of the present invention, it has been discovered that a stress analysis made on the basis of strain compatibility conditions that includes the composite nature of the waveguide core layer of planar optical waveguides provides a structure and technique for fabricating planar optical waveguides which more properly evaluate the strain in the waveguide core for the purpose of optimally reducing strain-induced birefringence. Preferably, the planar optical waveguides are uniquely characterized in that the xe2x80x9ceffectivexe2x80x9d coefficient of thermal expansion of the waveguide core layer is approximately the same as that of the substrate.